Multiple reference optical coherence tomography (MR-OCT) is a time-domain interferometric imaging platform which promises to be realized as a low cost, compact imaging technology. The optical configuration of MR-OCT makes use of multiple reflections between a partial mirror and the axially scanning reference mirror to enhance the axial imaging depth compared to the otherwise shallow scanning range of the voice coil or piezo actuator. Since each reflection on the partial mirror causes an optical path delay, the focusing of each higher order of reflection is changing. That means the beam diameter, beam collimation and therefore the interference visibility are changing as well. This investigation focuses on how the spacing between the partial mirror and the scanning mirror affects the system’s sensitivity and SNR values. The potential to improve the sensitivity of higher orders of reflections related to deeper regions in a sample will be studied.
In biomedical optics, Optical Coherence Tomography (OCT) is an emerging optical imaging modality during the last three decades. OCT can perform high resolution, cross-section imaging of the internal microstructure in biological tissues by measuring echoes of backscattered light. We have developed a broadband, high-resolution spectral domain OCT system whose central wavelength is 1300 nm with bandwidth ~400 nm. Theoretical axial resolution of the system is ~3.71 μm and experimentally we get ~5 μm in air. Furthermore, we will apply this system for nano-sensitive detection and visualization in varieties of biological tissues.
Corneal cross-linking (CXL) has grown from an interesting concept to a practical clinical treatment for corneal ectatic disease globally in the past three decades. In both understanding the principle of how CXL proceeds and monitoring the clinical procedure, detection of structural changes during cornea CXL plays a significant role. This paper demonstrates a novel over-sampling nano-sensitive optical coherence tomography (osnsOCT) method, which is potential to detect nanoscale structural changes in various tissues, to simultaneously measure the structural variations during the corneal CXL treatment.
The microscopy assisted by dielectric microspheres has emerged as an effective way to improve the imaging resolution of conventional optical microscope. The photonic nanojet(PNJ) is simulated by software, and super-resolution imaging experiments are carried out successfully. The results show that the silica(SiO2) microsphere with low refractive index can visualize a grating containing 139-nm-width lines with an interspacing of 139 nm under visible light illumination when it is partially immersed in alcohol. However, the barium titanate glass(BTG) microspheres with high refractive index can achieve super-resolution imaging only fully immersed in alcohol.
Three-dimensional measurement and inspection is an area with growing needs and interests in many domains, such as integrated circuits (IC), medical cure, and chemistry. Among the methods, broadband light interferometry is widely utilized due to its large measurement range, noncontact and high precision. In this paper, we propose a spatial modulation depth-based method to retrieve the surface topography through analyzing the characteristics of both frequency and spatial domains in the interferogram. Due to the characteristics of spatial modulation depth, the technique could effectively suppress the negative influences caused by light fluctuations and external disturbance. Both theory and experiments are elaborated to confirm that the proposed method can greatly improve the measurement stability and sensitivity with high precision. This technique can achieve a superior robustness with the potential to be applied in online topography measurement.
White-light scanning interferometry plays an important role in precise profile metrology of microstructure. However, applying this approach may also be limited because of the optical reflection behavior of the surface. While there is a thin film on the surface, the reflection behavior of top and bottom of the thin-film will cause severer phase errors. Recently, the method by combining both reflectometry and white-light scanning interferometry is proposed to measure the film thickness and surface profile. This article firstly explains the principle of the proposed method and then verifies the feasibility of the thickness-measurement method for transparent film on a Silicon surface. Both of the algorithm and the experiment system have been optimized to measure the film thickness with high precision.
Dimensional metrology for micro structure plays an important role in addressing quality issues and observing the performance of micro-fabricated products. Different from the traditional white-light interferometry approach, the modulation-based method is expected to measure topography of micro structure by the obtained modulation of each interferometry image. Through seeking the maximum modulation of every pixel respectively in Z direction, the method could obtain the corresponding height of individual pixel and finally get topography of the structure. Owing to the characteristic of modulation, the proposed method which is not influenced by the change of background light intensity caused by instable light source and different reflection index of the structure could be widely applied with high stability. The paper both illustrates the principle of this novel method and conducts the experiment to verify the feasibility.
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